1. Field of the Invention
The present invention relates generally to a conductive paste and a method of manufacturing the same, and is directed more particularly to a conductive paste suitable for use as a so-called through-hole filter for connecting the parts between the respective circuits on the respective layers of a multi-layer printed circuit board, a connecting material between conductive circuit patterns or making a conductive pattern and a method of manufacturing the same.
2. Description of the Prior Art
A prior art multi-layer printed circuit board will be described with reference to FIG. 1 which shows the manufacturing steps thereof in sequence. As shown in FIGS. 1A and 1B, a board 1 made of an insulating material such as phenolic resin, epoxy resin or the like is prepared, and copper foils 2 and 2' are respectively adhered on the upper and lower surface of the board 1 to thereby produce a so-called copper clad laminated board 3. Then, a through-hole 4 is bored at a predetermined location of the board 3 which passes through the board 3 and the copper foils 2, 2' thereon in the vertical direction. The board 3 is subjected to electroless copper plating and then to copper electro-plating to form a copper plating layer 5 on the inner surface of the through-hole 4 and on the surfaces of the copper foils 2 and 2' as shown in FIG. 1C. Then, an etching resist 6 is charged into the through-hole 4 and etching resists 7 are respectively printed on the copper foils 2 and 2' which have the patterns corresponding to the wiring patterns to be formed as shown in FIGS. 1D and 1E. Next, as shown in FIGS. 1F and 1G, the copper foils 2 and 2' are selectively etched with the resists 6 and 7 as a mask to remove unnecessary copper foil portions and hence to form wiring patterns 8 and 8', and then the resists 6 and 7 are removed to make a multi-layer printed circuit board 10.
Another example of the prior art multi-layer printed circuit board is shown in FIG. 3. In this example, a copper clad laminated board 3 similar to that 3 shown in FIG. 1 is first subjected to selective etching to remove unnecessary copper foil portions of copper foils 2, 2' and hence to form wiring patterns 8 and 8' as shown in FIGS. 3A and 3B. Then, a through-hole 4 is bored through the board 3 at a predetermined position which also passes through the wiring patterns 8 and 8', Ag paint 9 consisting of Ag powders and binder is charged into the through-hole 4 to electrically connect both wiring patterns 8 and 8' as shown in FIGS. 3C and 3D to manufacture a multi-layer printed circuit board 11.
The multi-layer printed circuit boards 10 and 11 shown in FIGS. 1 and 3 cannot be free from the following defects.
For example, in the case of the prior art multi-layer printed circuit board 10 shown in FIG. 1, when both wiring patterns 8 and 8' are electrically connected, the plating process and so on are necessary which is rather complicated, and low in yield. Further, the waste water disposal is not easy in view of public pollution. Also, since the copper plating is carried out on the entire surfaces of the board and thereafter the wiring patterns are selectively etched, unnecessary copper plating portion is rather much or useless copper plating is rather much. Further, as shown in FIG. 2, after the copper plating layer 5 is formed, due to an under-cut phenomenon 12 upon the etching, the wiring pattern 8 (or 8') becomes thin and accordingly, the pattern accuracy is deteriorated.
In the case of the multi-layer printed circuit board 11 shown in FIG. 3, when a DC voltage is applied to the board under humidification, Ag is moved from a high voltage portion to a low voltage portion and a short-circuit is caused which is so-called as a migration phenomenon. The through-hole resistance is higher than that shown in FIG. 1 by the copper plating by 100 times and hence the circuit board 11 is poor in high frequency characteristic. Further, since the Ag paint 9 charged into the through-hole 4 contains substantial amounts of binder and solvent, as shown in FIG. 4, the volume shrinkage of Ag paint 9 after curing by heat increases and hence the circuit board 11 is lacking in reliability. Further the use of solvent causes bad working conditions.